Watch a thrilling occultation of Spica on July 13 – Sky & Telescope

Lunar occultations are common, but they usually involve fainter, more telescopic stars. This month we have a wonderful exception. On the night of July 13, the Moon will cover Virgo's brightest star, Spica, across much of North and Central America. Sometimes the Moon's light overwhelms or diminishes the impact of an occultation. Fortunately, that won't be the case this time, because the Moon will be in its last quarter. Through a small telescope, you'll still be able to spot the outline of the Earth-lit western edge and preview the stunning moment of Spica's disappearance. Even binoculars will show the star's flash, though they may not be powerful enough to catch the light and reveal the edge.

Like a roundabout with no exits, the Moon's path is confined to the ecliptic zone. It goes around and around the circle, completing one revolution of the sky roughly every month. Because the Moon's orbit is tilted 5.1° relative to the plane of Earth's orbit (which defines the ecliptic), the Moon moves above and below the plane, so its path over time looks more like a ribbon than a circle. Within that band are four first-magnitude stars—Antares, Aldebaran, Regulus, and Spica—that occasionally lie directly in the Moon's path. When the Moon covers one of them, we can witness a relatively small, orbiting rock ball temporarily “remove” a star several thousand times its size from the sky.

Spica will disappear with surprising rapidity at the dark edge of the Moon and reappear later with the same abruptness at the opposite bright edge. Although the star appears solitary, it is actually a spectroscopic Binary system comprising a blue giant primary star nearly 8 times the diameter of the Sun and a secondary sun about half that size. Although it is theoretically possible to use video to record a gradual dimming of Spica's light as each star sets, the challenge is great. They are separated by one-third of the average distance between Mercury and the Sun. and They are 250 light-years away, factors that effectively shrink the duo to a point too small to be resolved even with the largest instruments.

Most stellar occultations happen suddenly for that reason. It takes a very short time for a moving object to cover a spotWhen combined with the Moon's lack of atmosphere (which would otherwise dim the star as it approached) and the Moon's speed at the time (3,500 kilometers per hour), it's easy to see why Spica would disappear in an instant.

Observers in the eastern states will see Spica disappear into the Moon’s dark edge around 10:30–11 p.m. local time. In the Midwest, the occultation occurs in full twilight around 10 o’clock, while in the Rocky Mountain Time Zone it occurs shortly after sunset. Although the Sun will still shine brightly on cities along the Pacific Coast, as long as the sky is clear of haze, Spica should be visible in daylight through a small telescope. Point your instrument toward the Moon and look for a point of light just to the east. With the dark edge invisible, Spica will disappear into a blue sky at the time of occultation. Stick around for the reappearance, too.

For information on dip and rise times for your city, visit the International Organization for Occultation Timing (IOTA) website. Spica Cache Page. Times are in UT or Universal Time. To convert to Eastern Daylight Time, subtract 4 hours and move the date back one day. For CDT, subtract 5 hours; 6 hours for MDT; and 7 hours for PDT. For example, Spica will disappear at 3:12 UT on July 14 from Lansing, Michigan. Subtract 4 hours (Eastern Time) to get 11:12 p.m. local time on July 13.

Spica's occultation season began on June 16, when the Moon covered the star for observers in Eastern Europe and northwestern Asia. Many more will follow until the current series ends on November 17, 2025. After that, the Moon will not disturb the pair again until 2031.

Update on comet 13P/Olbers

Comet 13P/Olbers, which I reported on here last month13P has been showing a slightly brighter glow than original expectations. In early July it shines at around magnitude 6.5 and is easily visible in 50mm binoculars in late twilight. On June 29, with my 10×50 binoculars, I noticed a diffuse coma about 5' in diameter with a brighter, more compact center and a tail tuft pointing northeast. In my 15-inch (38 cm) telescope, 13P's bright head and half-degree tail made for a beautiful sight.

As the comet crosses from Lynx toward the Big Dipper, it skirts the Three Leaps of the Gazelle asterism, three pairs of stars that represent the creature’s hoofprints in pond mud. The highly anticipated comet Tsuchinshan-ATLAS (C/2023 A3) has stalled around magnitude 10, though its tail has increased in length. It lies in southern Leo, very low in the southwestern sky at dusk. Observers at mid-northern latitudes have only a week or two before the comet disappears into the sun’s glare, not to be seen again until late September at dawn.